How does Zoro get his third sword in Onepiece?

Answer 1

Kuina dies and he asks his sensei for her sword.

Well, that's how he gets his trade mark sword, (wado) actually, we don't know how he aquires his 2 starting swords, but later in the seriers, (at louge) town he buts one of the 21 great swords, (it was cursed so he only paid 50 grand) and another of the 21 great swords is given to him by the store owner because he bought the cursed one.

Even later in the seriers, during the Ennis Lobby Incident, the sword give to him by the store owner, was rusted by a marine with the abilty to rust any metal. He reaquires a new third sword by defeating the zombie of an ancient samuri during the Thriller Bark episode.

Answer 2

Kuina died and Zoro asked for her sword. at the time, Zoro was already using the two-sword style, and in asking for Kuina's sword he had asked to carry her wish on along with his. in the sense that he couldn't give up his determination to make room for hers, he couldn't give up one of his sword for hers either. so he just added a third sword - the three-sword-style!! :D

Answer 3

Zoro receives his third sword, Sandai Kitetsu, as a gift from Ipponmatsu, a swordsmith, in Loguetown after his previous swords were destroyed. He later acquires a stronger version, Nidai Kitetsu, during the Wano Country arc.